Why is latent heat called latent heat



As latent heat ("latent" Latin for "hidden") is the name given to the amount of heat absorbed or released during a phase transition. It is called latent because the absorption or release of this heat does not lead to a change in temperature. Processes in which the supply of energy / transport of energy lead to a change in temperature are called sensible heat or sensible heat. Specific examples are the

  • Heat of evaporation (= heat of condensation) → phase transition from liquid to gas
  • Heat of fusion → phase transition from solid to liquid
  • Heat of crystallization → phase transition between amorphous and crystalline structure

The specific latent heat for each phase transition is tabulated - for the elements, see Periodic Table of the Elements. During the phase transition, heat is extracted or added to the substance without changing its temperature.

units

  • Joule - absolute energy (latent heat)
  • Joule / kilogram - based on mass ("specific latent heat")
  • Joule / Mol - based on the amount of substance ("molar latent heat")

Conversion into electron volts and Kelvin

In physics, the energy is usually not given per mole but per particle in electron volts [eV] or Kelvin [K]. 1 eV = 96.485 kJ / mol and 1 kJ / mol = 0.01036 eV.

root cause

The molecular structure of substances explains why there is no temperature change despite heat transport. Applied to the heat of evaporation, this means that the molecules in a liquid are much closer together than in a gas. During evaporation, the distance between the molecules must consequently be increased, which is associated with an increase in the potential energy. The work required for this is done by the amount of heat supplied. However, according to the kinetic gas theory, a positive temperature change corresponds to an increase in the kinetic energy of the molecules, which is not directly associated with an increase in distance.
The same reasoning also applies to the heat of fusion and the heat of crystallization.

importance

The latent heat plays an important role in meteorology in relation to the phase transitions of water in the earth's atmosphere. A large part of the solar energy is invested in the evaporation of water on a moist earth's surface or even water. 2257 kilojoules are converted per kilogram of water. There is no change in the air temperature, so the energy is stored in the gaseous state of the water, so to speak. Since this storage is reversible, the same amount of energy is released again when a rising air parcel reaches the condensation level and the water vapor condenses. The energy originally provided on the ground by solar radiation is released again at higher altitudes and contributes to an increase in temperature there. This leads to the formation of a moist adiabatic temperature gradient, so the atmosphere becomes much more leisurely upwards than would be expected with a dry adiabatic gradient without the latent heat.

See also

Specific heat capacity, Gibbs phase rule, latent heat storage, entropy

Category: Thermodynamics